1. Field of the Invention
The present invention relates to electrodes for electrochemical experiments. In particular, the present invention relates to reference electrodes that provide a high frequency response.
2. Discussion of Background
An electrochemical reference electrode, or "half-cell", typically uses the reversible conversion of either mercury or silver to the corresponding, sparingly water-soluble chloride to establish a current path between an aqueous solution and external circuitry. For example, a calomel electrode consists of a small amount of mercury surrounded by calomel (Hg.sub.2 Cl.sub.2) and potassium chloride (KCl) solution held in a glass tube. Electrical contact is made to the mercury via a platinum wire extending through the glass tube, and between the KCl solution in the tube and an external fluid through a porous plug that allows ion diffusion but no bulk flow. Small electric currents flowing between the platinum wire and the external fluid either reduce some of the calomel to metallic mercury or oxidize some of the mercury to calomel according to the following reaction: EQU Hg.sub.2 Cl.sub.2 +2e.sup.-.revreaction. 2Hg+2Cl.sup.-
Since the mercury-calomel (or silver-silver chloride) half-cell potentials are precisely known, and these electrodes therefore provide a "reference" potential, the potential developed by any other type of electrode (or sample) placed in the external fluid, which electrode is part of the other half-cell, can be found easily by subtracting the reference potential from the total. However, currents must be kept small if the potential is not to be obscured by solution resistance and quasi-resistive effects caused by finite ion diffusion speeds. Consequently, the impedance of a calomel electrode may range from a hundred ohms to several thousand ohms, depending on its construction and history of use.
Calomel and silver-chloride electrodes are classically used in measuring steady-state, D.C. voltages. However, in some research areas--for instance, in the study of corrosion films--additional data can be gathered by introducing a small amount of high-frequency A.C. and monitoring it along with the usual D.C. signal. Unfortunately, conventional laboratory amplifiers (potentiostats) generally have significant input capacitance which can shunt much of the high-frequency signal to ground, thus obscuring the high-frequency phenomena of interest.
Another type of electrode widely used in studying chemical systems is a bare platinum rod, wire or plate, directly immersed in the liquid under study. Such an electrode generates a voltage which varies with the relative concentrations of reduced and oxidized ions in the liquid; this is sometimes called the "redox" potential. Unlike the calomel and silver-chloride electrodes used for reference potential measurements, a platinum electrode has a very low impedance, since current flows easily between the bare metal of the electrode and the liquid in which it is immersed.
A method of overcoming the high-frequency limitations of standard reference electrodes was described by Mansfield, Lin, Chen, and Shih (J. Electrochem. Soc. 135, 906), in which the reference electrode is mounted side-by-side with a small platinum electrode and the two are coupled together by a capacitor. By blocking D.C. signals, the capacitor prevents the reference potential from being swamped by the much stronger (because of lower impedance) redox potential. At high frequencies, however, the capacitor allows extra signal power from the platinum electrode to flow over into the reference electrode circuit and hence to the potentiostat, largely overcoming the high-frequency losses present in the conventional reference electrodes.
While electrically favorable, the design of Mansfield, et al. still has disadvantages. If the distance between the two parts of the electrode is significant on the scale of the test apparatus, many factors can come into play to distort the test results. For example, if an A.C. signal flows in a direction parallel to the separation between the electrodes and is being attenuated--as is often the case where an external electrode is grounded or connected to the signal source--the two electrodes will see different levels of A. C. signal. Even small differences in electrode location can introduce large errors if there are significant changes in solution conductivity of dielectric constant from point to point: for instance, in systems which combine two different phases, such as a liquid and a gas or two immiscible liquids.